1. Field of the Invention
The present invention relates to polyolefin resin particles that contain a vinyl aromatic monomer that is polymerized in the polyolefin matrix to form an interpenetrating network of polyolefin-polymerized vinyl aromatic monomer, e.g. polystyrene. More particularly, the present invention relates to interpolymer resin particles that have little or no gel content; to a process for producing the interpolymer resin particles; to a polymer composition comprising the interpolymer resin particles and a second polymer; and to articles made from the interpolymer resin particles and/or from the aforesaid polymer composition. These articles may be formed via processing techniques, e.g. sheet extrusion, rotomolding, thermoforming, compression molding, injection molding, blown film extrusion, and direct-injection foamed sheet extrusion.
2. Background Art
It is known to polymerize vinyl aromatic monomers, such as styrene, in polyethylene. For example, U.S. Pat. No. 3,959,189 issued to Kitamori and assigned to Sekisui Kaseihin Kabushiki Kaisha, discloses a process for producing polyethylene resin particles. The polyethylene resin particles have a melt index (MI) value of 0.3 to 10 (Condition I, 190° C., 2.16 kg), a density of less than 0.93 g/cm3 and a VICAT softening point below 85° C. After the polyethylene resin particles are added to an aqueous suspension, 30% to 100% by weight based on the weight of the particles of a styrene monomer and a catalyst for polymerizing the monomer are added to the suspension, and the styrene monomer is polymerized therein. Embodiments include adding a cross-linking agent to the polyethylene prior to the polymerization and cross-linking the polyethylene before impregnating a blowing agent into the polyethylene resin particles to form foamable polyethylene resin particles. In view of the cross-linking agent, the polyethylene-polystyrene resin particles generally have high gel content, i.e. from about 10% to 45% by weight. The gel content of the final foamed shaped article can be as high as 60% to 80% by weight. Even though these cross-linked polyethylene-polystyrene resin foams may have superior thermal stability and toughness, these same characteristics make these resin particles unsuitable for use in processes such as compounding, extrusion processing, and injection molding since the cross-linking effect tends to reduce the melt flow of these particles which affects the processability of these particles in that the amperage needed to operate the processing machinery, e.g. extruder is increased. The melt fracture is therefore increased resulting in surface irregularities, such as holes and ridges.
A further example of polystyrene resin polymerized in polyethylene resin particles is described in Japanese Patent No. 32623/70. Cross-linking of the polyethylene resin, polymerization of styrene, and impregnation of the blowing agent are carried out at the same time. Since the polyethylene resin particles are cross-linked, the polyethylene resin particles generally contain a high gel content, i.e. at least about 24% by weight, making these polyethylene resin particles generally unsuitable for polymer processing, such as extrusion, injection molding, blown film, and direct injection foamed sheet extrusion. If the gel content is too high, hard spots form on the surface resulting in poor surface quality. The processing of the polymer proves to be difficult due to the high amperage needed for the machinery used to process the polymer, for example, extruders or injection molding machines.
U.S. Pat. No. 4,782,098 assigned to General Electric Co. discloses expandable interpolymer beads comprising polyphenylene ether resin and a polymerized vinyl aromatic monomer such as styrene. The vinyl aromatic monomer is polymerized in the presence of a polymerization catalyst to polymerize the vinyl aromatic monomer with the polyphenylene ether to form the interpolymer beads. Optionally, a cross-linking agent is added. A blowing agent is introduced under pressure into the thermoplastic resin beads. The cross-linking agent may be di-t-butyl peroxide, t-butyl cumyl peroxide, dicumyl peroxide, alpha, alpha-bis(t-butyl peroxy)p-di-isopropyl-benzene, 2,5-dimethyl-2,5-di(t-butyl peroxy)hexyne-3,2,5-dimethyl-2,5-di(benzoyl peroxy)hexane, and t-butyl peroxy isopropyl carbonate. Here again, when a cross-linking agent is used, the polyphenylene ether resin has a gel content of at least about 24% by weight. Since the morphology of the polyphenylene ether resin is amorphous, the resin generally may have poor environmental stress crack resistance (ESCR) properties.
U.S. Pat. Nos. 4,303,756 and 4,303,757 to Kajimura, et al. and assigned to Sekisui Kaseihin Kogyo Kabushiki Kaisha disclose a process for producing expandable thermoplastic resin beads. The process comprises suspending in an aqueous medium 20% to 70% by weight of a random copolymer of propylene and ethylene (U.S. Pat. No. 4,303,756) or polypropylene resin particles (U.S. Pat. No. 4,303,757), and 30% to 80% by weight of a vinyl aromatic monomer such as styrene; polymerizing the vinyl aromatic monomer in the presence of a polymerization catalyst to graft the vinyl aromatic monomer onto the backbone of polypropylene; and, optionally, adding a cross-linking agent to form graft-copolymerized thermoplastic resin beads; and introducing a blowing agent into the thermoplastic resin beads.
In general, the interpolymer resin particles of the above prior art generally are expandable thermoplastic resin particles having a high gel content of about 10% to 45% by weight in at least one instance and in the other instances about 24% by weight which limits the processability of the beads or particles when converted into articles such as solid sheet, film, etc. through processing techniques such as sheet extrusion, rotomolding, thermoforming, compression molding, injection molding, blown film extrusion, and direct-injection foamed sheet extrusion. Also, in general the interpolymer resin particles of the prior art are impregnated with a blowing agent in a further suspension process to form foamable or expandable particles that are used for foam moldings.
Expandable interpolymers of polypropylene and polyvinyl aromatic monomers are also disclosed in Kent D. Fudge U.S. Pat. Nos. 4,622,347; 4,677,134 and 4,692,471; and in Bartosiak et al U.S. Pat. No. 4,647,593, all of which are assigned to Atlantic Richfield Company. These interpolymers may be prepared according to the teachings of the aforesaid U.S. Pat. No. 4,303,756. The interpolymers are made expandable by impregnating them with a blowing agent. The particles are expanded under normal conditions for polystyrene particles to low density, fine cell structure foams by viscbreaking the interpolymers to a melt flow (Condition L) of at least double their original value and adding a lubricant and cell control agent while maintaining orientation stress in the final product at a minimum. Since the interpolymer particles of these aforesaid patents may be prepared according to U.S. Pat. No. 4,303,756, the interpolymer particles have the same limitations outlined in the above discussion of the '756 patent in that the interpolymer particles generally have a high gel content, i.e. about 24% by weight. These viscbroken interpolymer particles are impregnated with a blowing agent in a further suspension process to produce foamable or expandable particles for foam moldings.